1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to a complementary metal oxide semiconductor (CMOS) image sensor and a method for fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for preventing a back bias voltage from influencing a transistor formed on the outside of a photodiode in a CMOS image sensor sensing optical color sensitivity of the colors of red, green, and blue, which are included in light rays irradiated to the photodiode, without using any color filters.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device for converting an optical image into an electric signal. The image sensor can be broadly categorized into a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
The charge coupled device (CCD) includes a plurality of photodiodes (PD) aligned in a matrix form and converting light signals into electric signals, a plurality of vertical charge coupled devices (VCCD) formed between each vertical photodiode aligned in a matrix form and vertically transmitting electric charges generated from each photodiode, a horizontal charge coupled device (HCCD) horizontally transmitting the electric charges transmitted by each of the vertical charged couple devices (VCCD), and a sense amplifier sensing and outputting the horizontally transmitted electric charges.
However, the charged couple device (CCD) has the disadvantages of a complicated driving method, high power consumption, and a complicated fabrication process requiring a multi-phased photo process. Also, in the charge coupled device (CCD), a control circuit, a signal processing circuit, an analog to digital (A/D) converter circuit, and so on cannot be easily integrated into a charge coupled device chip, thereby having the problem of forming compact size products.
Recently, the complementary metal oxide semiconductor (CMOS) image sensor has been considered to be the next generation image sensor that can resolve the problems and disadvantages of the charge coupled device (CCD). The CMOS image sensor is a device adopting a CMOS technology using the control circuit, the signal processing circuit, and so on as a peripheral circuit, so as to form MOS transistors corresponding to the number of unit pixels on a semiconductor substrate, in order to sequentially detect the electric signals of each unit pixel by using a switching method, thereby representing an image.
Since the CMOS image sensor uses a CMOS fabrication technology, the CMOS image sensor is advantageous in that it has low power consumption and has a simple fabrication method through less photo process steps. Also, in the CMOS image sensor, a control circuit, a signal processing circuit, an A/D converter circuit, and so on can be integrated in a CMOS image sensor chip, thereby enabling the product to be fabricated in a compact size. Accordingly, the CMOS image sensor is currently being extensively used in various applied technologies, such as digital still cameras and digital video cameras.
A general CMOS image sensor will now be described with reference to the accompanying drawings.
FIG. 1 illustrates a schematic view of an equivalent circuit of one unit pixel in a general CMOS image sensor, and FIG. 2 illustrates a cross-sectional view of a related art CMOS image sensor.
Referring to FIG. 1, a unit pixel of a general CMOS image sensor is formed of one photodiode (PD) and three nMOS transistors (T1, T2, and T3). A cathode of the photodiode (PD) is integrated to a drain of a first nMOS transistor (T1) and a gate of a second NMOS transistor (T2). And, the sources of both first and second nMOS transistors (T1 and T2) are connected to a power line supplying a reference voltage (VR). The gate of the first nMOS transistor (T1) is connected to a reset line providing a reset signal (RST). Also, a source of a third NMOS transistor (T3) is connected to the drain of the second nMOS transistor (T2), the drain of the third nMOS transistor (T3) is connected to a reading circuit (not shown) through a signal line, and the gate of the third nMOS transistor (T3) is connected to a column select line providing a select signal (SLCT). Therefore, the first NMOS transistor (T1) is referred to as the reset transistor, the second nMOS transistor (T2) is referred to as a driving transistor, and the third nMOS transistor (T3) is referred to as a selecting transistor.
The structure of the CMOS image sensor will now be described in detail.
Referring to FIG. 2, a p-type epitaxial layer 11 having an active area and a field area defined thereon is formed on a p-type semiconductor substrate 10, and a field insulating laser (not shown) is formed in the field area. In the active area, an isolating barrier 13 is formed between a first region for forming the photodiode and a second region for forming the transistors. And, an n-type diffusion area is formed by ion injecting n-type impurities in the first region for forming the photodiode (PD) of the active area.
In addition, a gate electrode (G) and a gate insulator (GI) are formed in the second region of the p-type epitaxial layer 11. And, a source/drain (S/D) area is formed by ion-injecting impurities in the p-type epitaxial layer 11 on both sides of the gate electrode (G), thereby forming the transistor (TR). A transparent interlayer dielectric 17 is formed on an entire surface of the substrate including the photodiode (PD) and the transistor (TR). A color filter layer 18 is formed on the interlayer dielectric 17 formed above the photodiode (PD). A planarization layer 19 is formed on the interlayer dielectric 17 including the color filter layer 18. Then, a micro lens 21 is formed on the planarization layer 19 formed above the photodiode (PD).
Herein, the transistor (TR) is formed in an optical electric charge transmitter transmitting optical electric charges generated from the photodiode (PD). In the related art, only two transistors are aligned on the substrate 10 for simplicity, however, it is obvious to a person skilled in the art that a plurality of transistors is aligned on the substrate. Also, the semiconductor substrate 10 is formed of a p-type polycrystalline silicon substrate, and the color filter layer 18 is formed of photosensitive layers using red, green, and blue color dyes.
Generally, in the CMOS image sensor having the above-described structure, the color filter layer 18 is sequentially formed on the interlayer dielectric 17. More specifically, the photosensitive layer using a red color dye is coated by using a spin-coating process, exposed to light, and developed. The developed photosensitive layer is then remained on the color filter forming area in the interlayer dielectric 17, which is perpendicular to the photodiode (PD), and the photosensitive layer in the other areas is removed, thereby forming the color filter layer. By repeating the above-described method, the red, green, and blue color filter layers 18 are formed.
Therefore, in the related art, in order to form red, green, and blue color filter layers 18 having a color filter array, the coating, exposing, and developing processes should each be repeated three times, which not only complicates the fabrication process of the color filter array but also prevents the transmissivity of the red, green, and blue light rays each passing through the color filer layer from being uniformly maintained.
Recently, in order to resolve such problems of the color filter array, many alternative methods of sensing each of the red, green, and blue light rays without using the color filter have been proposed. Among the proposed methods, a method of using a micro prism is disclosed in the Korean Patent Application No. 10-2003-0056096. Also, a method of using a multiple slits is disclosed in the Korean Patent Application No. 10-2002-0039454: However, in the above-referenced methods, the fabrication processes of the micro prism and the multiple slits are very complicated and have many limitations in essentially resolving the problem of the complicated fabrication process of the CMOS image sensor.